Estuarine habitat rehabilitation measures

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Estuarine habitat rehabilitation involves interventions aimed at improving basic ecological functions impaired by human activities and enhancing a site's capacity to support ecological communities or provide ecosystem services. Important examples include re-establishing tidal exchange with formerly embanked land, modifying sediment dynamics and improving the condition of degraded estuarine sediments. Such interventions do not directly reconstruct an ecosystem; they modify habitat conditions under which biological communities can subsequently develop. Rehabilitation is less ambitious than restoration (see Principles of coastal habitat restoration), but is often a more feasible and realistic objective.


Re-establishing tidal exchange with embanked land

Fig. 1. Managed realignment typically involves creating one or more breaches in a coastal embankment, allowing tidal exchange with formerly reclaimed land. This differs from the natural development of an intertidal area.

Dike breaching

Managed realignment or managed retreat restores tidal exchange with land that was formerly embanked for agriculture or other uses. This is generally achieved by locally breaching the seawall or dike, which is less expensive than removing the entire structure (Garbutt et al., 2006[1]). The dimensions and location of the breach determine tidal exchange and strongly influence subsequent morphological and ecological development.

Cost-benefit analyses have shown advantages of managed realignment over constructed coastal defence options in appropriate settings (Turner et al., 2007[2]). An overview of managed realignment projects implemented up to 2014 is given by Esteves (2014[3]).

Sedimentation and morphological development

The design of the tidal connection strongly influences sedimentation and morphological development in the reconnected area. An adjustable inlet can regulate sedimentation rates and thereby influence creek development (Oosterlee et al., 2020[4]).

The origin of the deposited sediment must also be considered. Where sediment supply is limited, deposition in the reconnected area can alter the sediment balance of the estuary and enhance erosion of intertidal areas or flood defences elsewhere (Morris, 2012[5]).

Fig. 2. Tidal flats are densely populated with organisms such as worms and bivalves that support the estuarine and marine food web. Illustration J-C. Goubert.

Intertidal habitat development

Tidal reconnection creates new intertidal area, but its subsequent development depends strongly on sedimentation. In turbid estuaries, rapid deposition in sheltered reconnected areas can raise the bed and promote succession from mudflat to salt marsh. Mudflats are an important estuarine habitat. They host a large number of organisms that provide a rich food source for fish and birds (Fig. 2). Strong microbial activity in the sediment top layer fulfills an important water purification function.[6] The area of mudflats along European coasts has strongly diminished during past centuries (EEA, 2015[7]). Where the management objective is to maintain unvegetated intertidal habitat, continued intervention such as sediment removal or redistribution may be necessary to counteract sedimentation and succession toward salt marsh (Pontee, 2014[8]). Such intervention represents habitat management rather than restoration of natural morphological processes.

Legacy of former land use

Re-establishing tidal exchange does not remove the effects of former land use. Reconnected areas can therefore follow a different morphological and ecological trajectory from natural intertidal areas (Hazelden and Boorman, 2001[9]; Mazik et al., 2010[10]). Former agricultural use can leave a particularly persistent soil legacy. Compaction by livestock and agricultural machinery produces poorly permeable soils beneath newly deposited sediment, restricting drainage and potentially affecting vegetation development and nutrient cycling (Van Putte et al., 2020[11]). Biological communities in reconnected salt marshes can consequently differ from those at natural reference sites (Mossman et al., 2012[12]). Where former land use has fundamentally altered site conditions, tidal reconnection may therefore produce a new intertidal habitat rather than reproduce a historical reference state (Gerwing et al., 2020[13]). Possible rehabilitation measures include deep ploughing, increasing soil organic matter and creating small creeks and topographic variation (Brooks et al., 2015; Lawrence et al., 2018).

Sand-capping of organic-enriched estuarine sediments

Long-term eutrophication can leave estuaries with organic-enriched muddy sediments trapped in the estuarine turbidity maximum. Cycles of resuspension, mineralization, primary production and sedimentation prolong the eutrophication status. The resulting turbidity and oxygen depletion impede recovery of benthic fauna and light-dependent macrophytes[14][15]. Recovery can therefore remain slow even after external nutrient loading has been substantially reduced.

Organic-enriched mud can be removed by dredging, but a less costly rehabilitation measure in low-energy environments is capping with clean sand sufficiently coarse to resist resuspension. Besides reducing resuspension of organic mud, the cap modifies sediment texture and biogeochemical conditions. Sand capping was originally developed mainly to isolate contaminated sediments, but the technique can also be applied to rehabilitation of organic-enriched estuarine sediments. A sand cap experiment was conducted in Denmark's microtidal Odense Fjord, a shallow low-energy basin with organic-enriched mud deposits that are frequently stirred up if no sand cap is applied[16]. This eutrophicated system formerly supported extensive eelgrass meadows that disappeared during the last decades of the twentieth century and have not recovered despite a substantial reduction in nutrient input. An evaluation after 12 months showed that the 10 cm thick sand cap was effective in reducing the turbidity and increase light penetration to the bottom. The benthic biodiversity increased and the sand cap improved the anchoring capacity of eelgrass in comparison with the fine grained mud. However, mud from uncapped areas settled on the capped area; better results can be expected if the scale of the experiment had been enlarged to include also other mud deposits.


Related articles

Principles of coastal habitat restoration
Dynamics, threats and management of salt marshes
Spatial and temporal variability of salt marshes
Nature-based shore protection
Climate adaptation measures for the coastal zone
Integrated Coastal Zone Management (ICZM)


References

  1. ↑ Garbutt, R.A., Reading, C.J., Wolters, M., Gray, A.J. and Rothery, P. 2006. Monitoring the development of intertidal habitats on former agricultural land after the managed realignment of coastal defences at Tollesbury, Essex, UK. Marine Pollution Bulletin 53: 155-164.
  2. ↑ Turner, R.K., Burgess, D., Hadley, D., Coombes, E. and Jackson, N. 2007. A cost-benefit appraisal of coastal managed realignment policy. Global Environmental Change 17: 397-407.
  3. ↑ Esteves, L.S. 2014. Managed realignment: A viable long-term coastal management strategy? Springer Briefs in Environmental Science, Springer, New York, 143 pp.
  4. ↑ Oosterlee, L., Cox, T.J.S., Temmerman, S. and Meire, P. 2020. Effects of tidal re-introduction design on sedimentation rates in previously embanked tidal marshes. Estuarine, Coastal and Shelf Science 244, 106428.
  5. ↑ Morris, R.K.A. 2012. Managed realignment: A sediment management perspective. Ocean & Coastal Management 65: 59-66.
  6. ↑ Elliott, M. and Whitfield, A.K. 2011. Challenging paradigms in estuarine ecology and management. Estuarine, Coastal and Shelf Science 94: 306-314
  7. ↑ EEA 2015. Report under the Article 17 of the Habitats Directive Period 2007-2012 1140 Mudflats and sandflats not covered by sea water at low tide. European Environment Agency European Topic Centre on Biological Diversity
  8. ↑ Pontee, N. 2014. Accounting for siltation in the design of intertidal creation schemes. Ocean & Coastal Management 88: 8-12.
  9. ↑ Hazelden, J. and Boorman, L.A. 2001. Soils and 'managed retreat' in South East England. Soil Use and Management 17: 150-154.
  10. ↑ Mazik, K., Musk, W., Dawes, O., Solyanko, K., Brown, S., Mander, L. and Elliott, M. 2010. Managed realignment as compensation for the loss of intertidal mudflat: a short term solution to a long term problem? Estuarine, Coastal and Shelf Science 90: 11-20.
  11. ↑ Van Putte, N., Temmerman, S., Verreydt, G., Seuntjens, P., Maris, T., Heyndrickx, M., Boone, M., Joris, I. and Meire, P. 2020. Groundwater dynamics in a restored tidal marsh are limited by historical soil compaction. Estuarine, Coastal and Shelf Science 244, 106101.
  12. ↑ Mossman, H.L., Davy, A.J. and Grant, A. 2012. Does managed coastal realignment create saltmarshes with ‘equivalent biological characteristics’ to natural reference sites? Journal of Applied Ecology 49: 1446-1456.
  13. ↑ Gerwing, T.G. et al. 2020. Do you want to breach an embankment? Synthesis of the literature and practical considerations for breaching of tidally influenced causeways and dikes. Estuarine, Coastal and Shelf Science 245, 107024.
  14. ↑ Karlson, K., Rosenber, R., Bonsdorff, E., 2002. Temporal and spatial large-scale effects of eutrophication and oxygen deficiency on benthic fauna in Scandinavian and Baltic waters. Oceanogr. Mar. Biol. 40, 427–489
  15. ↑ Levin, L.A., Ekau, W., Gooday, A.J., Jorissen, F., Middelburg, J.J., Naqvi, S.W.A., Neira, C., Rabalais, N.N., Zhang, J., 2009. Effects of natural and human-induced hypoxia on coastal benthos. Biogeosciences 6, 2063–2098
  16. ↑ Oncken, N.S., Lange, T., Kristensen, E., Quintana, C.O., Steinfurth, R.C. and Flindt, M.R. 2022. Sand-capping – A large-scale approach to restore organic-enriched estuarine sediments. Marine Environmental Research 173, 105534


The main authors of this article are van Belzen, Jim, Bouma, Tjeerd, Skov, Martin, Zhang, Liquan and Yuan, Lin
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